Intra-prediction for smooth blocks in image/video

ABSTRACT

Coding a block of a video frame using an intra-prediction mode is disclosed. A method includes selecting first neighboring pixels, generating second pixels for use along a second edge that is opposite the first edge of the block, and generating a prediction block that includes predicted pixels. The first neighboring pixels are peripheral to the block along a first edge of the block. The second pixels are generated using third neighboring pixels that are peripheral to a third edge of the block. The third edge is different from the first edge and the second edge. Generating the prediction block includes interpolating, using a first interpolation, the first neighboring pixels and the second pixels to obtain the predicted pixels.

BACKGROUND

Digital video streams may represent video using a sequence of frames orstill images. Digital video can be used for various applications,including, for example, video conferencing, high-definition videoentertainment, video advertisements, or sharing of user-generatedvideos. A digital video stream can contain a large amount of data andconsume a significant amount of computing or communication resources ofa computing device for processing, transmission, or storage of the videodata. Various approaches have been proposed to reduce the amount of datain video streams, including compression and other encoding techniques.

SUMMARY

One aspect of the disclosed implementations is a method for coding ablock of a video frame using an intra-prediction mode. The methodincludes selecting first neighboring pixels, generating second pixelsfor use along a second edge that is opposite the first edge of theblock, and generating a prediction block that includes predicted pixels.The first neighboring pixels are peripheral to the block along a firstedge of the block. The second pixels are generated using thirdneighboring pixels that are peripheral to a third edge of the block. Thethird edge is different from the first edge and the second edge.Generating the prediction block includes interpolating, using a firstinterpolation, the first neighboring pixels and the second pixels toobtain the predicted pixels.

Another aspect is an apparatus for encoding a block of a video frameusing an intra-prediction mode that includes a memory and a processor.The processor is configured to execute instructions stored in the memoryto select first neighboring pixels, generate second pixels for use alonga second edge that is opposite the first edge of the block, generate aprediction block that includes predicted pixels, generate a residualblock using the prediction block and the block of video, and encode theresidual block in a compressed bitstream. The first neighboring pixelsare peripheral to the block along a first edge of the block. The secondpixels are generated using third neighboring pixels, which areperipheral to a third edge of the block. The third edge is differentfrom the first edge and the second edge. The prediction block isgenerated by interpolating, using an interpolation, the firstneighboring pixels and the second pixels to obtain the predicted pixels.

Another aspect is an apparatus for decoding a block of a video frameusing an intra-prediction mode that includes a memory and a processor.The processor is configured to execute instructions stored in the memoryto select first neighboring pixels, the first neighboring pixels beingperipheral to the block along a first edge of the block; generate secondpixels for use along a second edge that is opposite the first edge ofthe block, the second pixels generated using third neighboring pixels,the third neighboring pixels being peripheral to a third edge of theblock, and the third edge being different from the first edge and thesecond edge; generate a prediction block comprising predicted pixels byinstructions to interpolate, using an interpolation, the firstneighboring pixels and the second pixels to obtain the predicted pixels;decode, from a compressed bitstream, a residual block; and reconstructthe block using the residual block and the prediction block.

These and other aspects of the present disclosure are disclosed in thefollowing detailed description of the embodiments, the appended claimsand the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The description herein makes reference to the accompanying drawingswherein like reference numerals refer to like parts throughout theseveral views.

FIG. 1 is a schematic of a video encoding and decoding system.

FIG. 2 is a block diagram of an example of a computing device that canimplement a transmitting station or a receiving station.

FIG. 3 is a diagram of a video stream to be encoded and subsequentlydecoded.

FIG. 4 is a block diagram of an encoder according to implementations ofthis disclosure.

FIG. 5 is a block diagram of a decoder according to implementations ofthis disclosure.

FIG. 6A is a diagram of directional intra-prediction modes according toimplementations of this disclosure.

FIG. 6B is a diagram of examples of intra-prediction modes according toimplementations of this disclosure.

FIG. 7 is a diagram of examples of smooth intra-prediction modesaccording to implementations of this disclosure.

FIG. 8 is an example of weights used for a quadratic interpolationaccording to implementations of this disclosure.

FIG. 9 is a flowchart diagram of a process for intra-coding a currentblock according to an implementation of this disclosure.

FIG. 10 is a flowchart diagram of a process for coding a current blockusing an intra-prediction mode according to an implementation of thisdisclosure.

FIG. 11 is a flowchart diagram of a process for coding a block of avideo frame using an intra-prediction mode according to animplementation of this disclosure.

DETAILED DESCRIPTION

As mentioned above, compression schemes related to coding video streamsmay include breaking images into blocks and generating a digital videooutput bitstream (i.e., an encoded bitstream) using one or moretechniques to limit the information included in the output bitstream. Areceived bitstream can be decoded to re-create the blocks and the sourceimages from the limited information. Encoding a video stream, or aportion thereof, such as a frame or a block, can include using spatialsimilarities in the video stream to improve coding efficiency. Forexample, a current block of a video stream may be encoded based onidentifying a difference (residual) between the previously coded pixelvalues, or between a combination of previously coded pixel values, andthose in the current block.

Encoding using spatial similarities is referred to as intra-prediction.Intra-prediction techniques exploit spatial redundancy within a videoframe for compression. Specifically, an image block (e.g., a block of astill image or a block of a frame of a video) can be predicted usingneighboring coded pixels. An image block being coded is referred to as acurrent block. The neighboring pixels are typically pixels ofreconstructed blocks of previously coded blocks. The previously codedblocks are blocks that precede the current block in a scan order ofblocks of the image. For example, in a raster scan order, thereconstructed blocks are located on the top boundary and the leftboundary, but outside, of the current block.

Intra-prediction attempts to predict the pixel values of the currentblock of a frame of video using pixels peripheral to the block; that is,using pixels that are in the same frame as the block but that areoutside the block. A codec typically includes (e.g., implements) severalintra-prediction modes. An intra-prediction mode may be selected by theencoder as part of a rate distortion loop. In brief, variousintra-prediction modes may be tested to determine which type ofprediction will have the lowest distortion for a given rate, or numberof bits to be transmitted in an encoded video bitstream, includingoverhead bits included in the bitstream to indicate the type ofprediction used. As further described below with respect to FIGS. 6A-6Band FIG. 7, intra-prediction modes can be directional ornon-directional.

Implementations according to this disclosure can use one or moreintra-prediction modes that are useful for predicting blocks that havesmooth gradients. A smooth gradient refers to the gradual and continuouschange in the intensity or color in the current block. The gradient canbe in the horizontal direction, the vertical direction, or both.

Implementations according to this disclosure use differentintra-prediction modes depending on the gradient direction. For example,a first intra-prediction mode (referred to herein as the “SMOOTH_H_PRED”mode) may be selected by a codec when the gradient of the current blockis primarily or substantially horizontal; a second intra-prediction mode(referred to herein as the “SMOOTH_V_PRED” mode) may be selected by thecodec when the gradient of the current block is primarily orsubstantially vertical; and a third intra-prediction mode (referred toherein as the “SMOOTH_PRED” mode) may be selected by a codec when thegradient of the current block is primarily or substantially in both thevertical and the horizontal directions. Implementations of thisdisclosure can result in higher compression ratios for blocks that havesmooth gradients.

Intra-prediction for smooth image and/or video blocks is describedherein first with reference to a system in which the teachings may beincorporated.

FIG. 1 is a schematic of a video encoding and decoding system 100. Atransmitting station 102 can be, for example, a computer having aninternal configuration of hardware, such as that described in FIG. 2.However, other suitable implementations of the transmitting station 102are possible. For example, the processing of the transmitting station102 can be distributed among multiple devices.

A network 104 can connect the transmitting station 102 and a receivingstation 106 for encoding and decoding of the video stream. Specifically,the video stream can be encoded in the transmitting station 102, and theencoded video stream can be decoded in the receiving station 106. Thenetwork 104 can be, for example, the Internet. The network 104 can alsobe a local area network (LAN), wide area network (WAN), virtual privatenetwork (VPN), cellular telephone network, or any other means oftransferring the video stream from the transmitting station 102 to, inthis example, the receiving station 106.

The receiving station 106, in one example, can be a computer having aninternal configuration of hardware, such as that described in FIG. 2.However, other suitable implementations of the receiving station 106 arepossible. For example, the processing of the receiving station 106 canbe distributed among multiple devices.

Other implementations of the video encoding and decoding system 100 arepossible. For example, an implementation can omit the network 104. Inanother implementation, a video stream can be encoded and then storedfor transmission at a later time to the receiving station 106 or anyother device having memory. In one implementation, the receiving station106 receives (e.g., via the network 104, a computer bus, and/or somecommunication pathway) the encoded video stream and stores the videostream for later decoding. In an example implementation, a real-timetransport protocol (RTP) is used for transmission of the encoded videoover the network 104. In another implementation, a transport protocolother than RTP (e.g., an HTTP-based video streaming protocol) may beused.

When used in a video conferencing system, for example, the transmittingstation 102 and/or the receiving station 106 may include the ability toboth encode and decode a video stream as described below. For example,the receiving station 106 could be a video conference participant whoreceives an encoded video bitstream from a video conference server(e.g., the transmitting station 102) to decode and view and furtherencodes and transmits its own video bitstream to the video conferenceserver for decoding and viewing by other participants.

FIG. 2 is a block diagram of an example of a computing device 200 thatcan implement a transmitting station or a receiving station. Forexample, the computing device 200 can implement one or both of thetransmitting station 102 and the receiving station 106 of FIG. 1. Thecomputing device 200 can be in the form of a computing system includingmultiple computing devices, or in the form of a single computing device,for example, a mobile phone, a tablet computer, a laptop computer, anotebook computer, a desktop computer, and the like.

A CPU 202 in the computing device 200 can be a central processing unit.Alternatively, the CPU 202 can be any other type of device, or multipledevices, now-existing or hereafter developed, capable of manipulating orprocessing information. Although the disclosed implementations can bepracticed with a single processor as shown, for example, the CPU 202,advantages in speed and efficiency can be achieved by using more thanone processor.

In an implementation, a memory 204 in the computing device 200 can be aread-only memory (ROM) device or a random-access memory (RAM) device.Any other suitable type of storage device can be used as the memory 204.The memory 204 can include code and data 206 that is accessed by the CPU202 using a bus 212. The memory 204 can further include an operatingsystem 208 and application programs 210, the application programs 210including at least one program that permits the CPU 202 to perform themethods described herein. For example, the application programs 210 caninclude applications 1 through N, which further include a video codingapplication that performs the methods described herein. The computingdevice 200 can also include a secondary storage 214, which can, forexample, be a memory card used with a computing device 200 that ismobile. Because the video communication sessions may contain asignificant amount of information, they can be stored in whole or inpart in the secondary storage 214 and loaded into the memory 204 asneeded for processing.

The computing device 200 can also include one or more output devices,such as a display 218. The display 218 may be, in one example, atouch-sensitive display that combines a display with a touch-sensitiveelement that is operable to sense touch inputs. The display 218 can becoupled to the CPU 202 via the bus 212. Other output devices that permita user to program or otherwise use the computing device 200 can beprovided in addition to or as an alternative to the display 218. Whenthe output device is or includes a display, the display can beimplemented in various ways, including as a liquid crystal display(LCD); a cathode-ray tube (CRT) display; or a light emitting diode (LED)display, such as an organic LED (OLED) display.

The computing device 200 can also include or be in communication with animage-sensing device 220, for example, a camera, or any otherimage-sensing device, now existing or hereafter developed, that cansense an image, such as the image of a user operating the computingdevice 200. The image-sensing device 220 can be positioned such that itis directed toward the user operating the computing device 200. In anexample, the position and optical axis of the image-sensing device 220can be configured such that the field of vision includes an area that isdirectly adjacent to the display 218 and from which the display 218 isvisible.

The computing device 200 can also include or be in communication with asound-sensing device 222, for example, a microphone, or any othersound-sensing device, now existing or hereafter developed, that cansense sounds near the computing device 200. The sound-sensing device 222can be positioned such that it is directed toward the user operating thecomputing device 200 and can be configured to receive sounds, forexample, speech or other utterances, made by the user while the useroperates the computing device 200.

Although FIG. 2 depicts the CPU 202 and the memory 204 of the computingdevice 200 as being integrated into a single unit, other configurationscan be utilized. The operations of the CPU 202 can be distributed acrossmultiple machines (each machine having one or more processors) that canbe coupled directly or across a local area or other network. The memory204 can be distributed across multiple machines, such as a network-basedmemory or memory in multiple machines performing the operations of thecomputing device 200. Although depicted here as a single bus, the bus212 of the computing device 200 can be composed of multiple buses.Further, the secondary storage 214 can be directly coupled to the othercomponents of the computing device 200 or can be accessed via a networkand can comprise a single integrated unit, such as a memory card, ormultiple units, such as multiple memory cards. The computing device 200can thus be implemented in a wide variety of configurations.

FIG. 3 is a diagram of an example of a video stream 300 to be encodedand subsequently decoded. The video stream 300 includes a video sequence302. At the next level, the video sequence 302 includes a number ofadjacent frames 304. While three frames are depicted as the adjacentframes 304, the video sequence 302 can include any number of adjacentframes 304. The adjacent frames 304 can then be further subdivided intoindividual frames, for example, a frame 306. At the next level, theframe 306 can be divided into a series of segments 308 or planes. Thesegments 308 can be subsets of frames that permit parallel processing,for example. The segments 308 can also be subsets of frames that canseparate the video data into separate colors. For example, the frame 306of color video data can include a luminance plane and two chrominanceplanes. The segments 308 may be sampled at different resolutions.

Whether or not the frame 306 is divided into the segments 308, the frame306 may be further subdivided into blocks 310, which can contain datacorresponding to, for example, 16×16 pixels in the frame 306. The blocks310 can also be arranged to include data from one or more segments 308of pixel data. The blocks 310 can also be of any other suitable size,such as 4×4 pixels, 8×8 pixels, 16×8 pixels, 8×16 pixels, 16×16 pixelsor larger.

FIG. 4 is a block diagram of an encoder 400 in accordance withimplementations of this disclosure. The encoder 400 can be implemented,as described above, in the transmitting station 102, such as byproviding a computer software program stored in memory, for example, thememory 204. The computer software program can include machineinstructions that, when executed by a processor, such as the CPU 202,cause the transmitting station 102 to encode video data in mannersdescribed herein. The encoder 400 can also be implemented as specializedhardware included in, for example, the transmitting station 102. Theencoder 400 has the following stages to perform the various functions ina forward path (shown by the solid connection lines) to produce anencoded or compressed bitstream 420 using the video stream 300 as input:an intra/inter-prediction stage 402, a transform stage 404, aquantization stage 406, and an entropy encoding stage 408. The encoder400 may also include a reconstruction path (shown by the dottedconnection lines) to reconstruct a frame for encoding of future blocks.In FIG. 4, the encoder 400 has the following stages to perform thevarious functions in the reconstruction path: a dequantization stage410, an inverse transform stage 412, a reconstruction stage 414, and aloop filtering stage 416. Other structural variations of the encoder 400can be used to encode the video stream 300.

When the video stream 300 is presented for encoding, the frame 306 canbe processed in units of blocks. At the intra/inter-prediction stage402, a block can be encoded using intra-frame prediction (also calledintra-prediction) or inter-frame prediction (also calledinter-prediction), or a combination of both. In any case, a predictionblock can be formed. In the case of intra-prediction, all or a part of aprediction block may be formed from samples in the current frame thathave been previously encoded and reconstructed. In the case ofinter-prediction, all or part of a prediction block may be formed fromsamples in one or more previously constructed reference framesdetermined using motion vectors.

Next, still referring to FIG. 4, the prediction block can be subtractedfrom the current block at the intra/inter-prediction stage 402 toproduce a residual block (also called a residual). The transform stage404 transforms the residual into transform coefficients in, for example,the frequency domain using block-based transforms. Such block-basedtransforms (i.e., transform types) include, for example, the DiscreteCosine Transform (DCT) and the Asymmetric Discrete Sine Transform(ADST). Other block-based transforms are possible. Further, combinationsof different transforms may be applied to a single residual. In oneexample of application of a transform, the DCT transforms the residualblock into the frequency domain where the transform coefficient valuesare based on spatial frequency. The lowest frequency (DC) coefficient isat the top-left of the matrix, and the highest frequency coefficient isat the bottom-right of the matrix. It is worth noting that the size of aprediction block, and hence the resulting residual block, may bedifferent from the size of the transform block. For example, theprediction block may be split into smaller blocks to which separatetransforms are applied.

The quantization stage 406 converts the transform coefficients intodiscrete quantum values, which are referred to as quantized transformcoefficients, using a quantizer value or a quantization level. Forexample, the transform coefficients may be divided by the quantizervalue and truncated. The quantized transform coefficients are thenentropy encoded by the entropy encoding stage 408. Entropy coding may beperformed using any number of techniques, including token and binarytrees. The entropy-encoded coefficients, together with other informationused to decode the block (which may include, for example, the type ofprediction used, transform type, motion vectors, and quantizer value),are then output to the compressed bitstream 420. The information todecode the block may be entropy coded into block, frame, slice, and/orsection headers within the compressed bitstream 420. The compressedbitstream 420 can also be referred to as an encoded video stream orencoded video bitstream, and the terms will be used interchangeablyherein.

The reconstruction path in FIG. 4 (shown by the dotted connection lines)can be used to ensure that both the encoder 400 and a decoder 500(described below) use the same reference frames and blocks to decode thecompressed bitstream 420. The reconstruction path performs functionsthat are similar to functions that take place during the decodingprocess that are discussed in more detail below, including dequantizingthe quantized transform coefficients at the dequantization stage 410 andinverse transforming the dequantized transform coefficients at theinverse transform stage 412 to produce a derivative residual block (alsocalled a derivative residual). At the reconstruction stage 414, theprediction block that was predicted at the intra/inter-prediction stage402 can be added to the derivative residual to create a reconstructedblock. The loop filtering stage 416 can be applied to the reconstructedblock to reduce distortion, such as blocking artifacts.

Other variations of the encoder 400 can be used to encode the compressedbitstream 420. For example, a non-transform based encoder 400 canquantize the residual signal directly without the transform stage 404for certain blocks or frames. In another implementation, an encoder 400can have the quantization stage 406 and the dequantization stage 410combined into a single stage.

FIG. 5 is a block diagram of a decoder 500 in accordance withimplementations of this disclosure. The decoder 500 can be implementedin the receiving station 106, for example, by providing a computersoftware program stored in the memory 204. The computer software programcan include machine instructions that, when executed by a processor,such as the CPU 202, cause the receiving station 106 to decode videodata in the manners described below. The decoder 500 can also beimplemented in hardware included in, for example, the transmittingstation 102 or the receiving station 106.

The decoder 500, similar to the reconstruction path of the encoder 400discussed above, includes in one example the following stages to performvarious functions to produce an output video stream 516 from thecompressed bitstream 420: an entropy decoding stage 502, adequantization stage 504, an inverse transform stage 506, anintra/inter-prediction stage 508, a reconstruction stage 510, a loopfiltering stage 512, and a post filtering stage 514. Other structuralvariations of the decoder 500 can be used to decode the compressedbitstream 420.

When the compressed bitstream 420 is presented for decoding, the dataelements within the compressed bitstream 420 can be decoded by theentropy decoding stage 502 to produce a set of quantized transformcoefficients. The dequantization stage 504 dequantizes the quantizedtransform coefficients (e.g., by multiplying the quantized transformcoefficients by the quantizer value), and the inverse transform stage506 inverse transforms the dequantized transform coefficients using theselected transform type to produce a derivative residual that can beidentical to that created by the inverse transform stage 412 in theencoder 400. Using header information decoded from the compressedbitstream 420, the decoder 500 can use the intra/inter-prediction stage508 to create the same prediction block as was created in the encoder400, for example, at the intra/inter-prediction stage 402. At thereconstruction stage 510, the prediction block can be added to thederivative residual to create a reconstructed block. The loop filteringstage 512 can be applied to the reconstructed block to reduce blockingartifacts. Other filtering can be applied to the reconstructed block. Inan example, the post filtering stage 514 is applied to the reconstructedblock to reduce blocking distortion, and the result is output as anoutput video stream 516. The output video stream 516 can also bereferred to as a decoded video stream, and the terms will be usedinterchangeably herein.

Other variations of the decoder 500 can be used to decode the compressedbitstream 420. For example, the decoder 500 can produce the output videostream 516 without the post filtering stage 514. In some implementationsof the decoder 500, the post filtering stage 514 is applied after theloop filtering stage 512. The loop filtering stage 512 can include anoptional deblocking filtering stage. Additionally, or alternatively, theencoder 400 includes an optional deblocking filtering stage in the loopfiltering stage 416.

A codec can use multiple transform types. For example, a transform typecan be the transform type used by the transform stage 404 of FIG. 4 togenerate the transform block. For example, the transform type (i.e., aninverse transform type) can be the transform type to be used by thedequantization stage 504 of FIG. 5. Available transform types caninclude a one-dimensional Discrete Cosine Transform (1D DCT) or itsapproximation, a one-dimensional Discrete Sine Transform (1D DST) or itsapproximation, a two-dimensional DCT (2D DCT) or its approximation, atwo-dimensional DST (2D DST) or its approximation, and an identitytransform. Other transform types can be available. In an example, aone-dimensional transform (1D DCT or 1D DST) can be applied in onedimension (e.g., row or column) and the identity transform applied inthe other dimension.

In the cases where a 1D transform (e.g., 1D DCT, 1D DST) is used (e.g.,1D DCT is applied to columns (or rows, respectively) of a transformblock), the quantized coefficients can be coded by using a row-by-row(i.e., raster) scanning order or a column-by-column scanning order. Inthe cases where 2D transforms (e.g., 2D DCT) are used, a differentscanning order may be used to code the quantized coefficients. Asindicated above, different templates can be used to derive contexts forcoding the non-zero flags of the non-zero map based on the types oftransforms used. As such, in an implementation, the template can beselected based on the transform type used to generate the transformblock. As indicated above, examples of a transform type include: 1D DCTapplied to rows (or columns) and an identity transform applied tocolumns (or rows); 1D DST applied to rows (or columns) and an identitytransform applied to columns (or rows); 1D DCT applied to rows (orcolumns) and 1D DST applied to columns (or rows); a 2D DCT; and a 2DDST. Other combinations of transforms can comprise a transform type.

As mentioned above, a current block can be predicted usingintra-prediction. An intra-prediction mode uses pixels peripheral to thecurrent block being predicted. Pixels peripheral to the current blockare pixels outside the current block. Many different intra-predictionmodes can be available. Some intra-prediction modes, which may bereferred to as directional intra-prediction modes, can have acorresponding prediction angle. Other intra-prediction modes arenon-directional. Other types of intra-prediction modes can also beavailable.

In an example, the following 13 intra-prediction modes can be available:DC_PRED, V_PRED, H_PRED, D45_PRED, D135_PRED, D117_PRED, D153_PRED,D207_PRED, D63_PRED, SMOOTH_PRED, SMOOTH_V_PRED, SMOOTH_H_PRED, andPAETH_PRED.

FIG. 6A is a diagram 600 of directional intra-prediction modes accordingto implementations of this disclosure. FIG. 6A illustrates groupingdirectional intra-prediction modes into three zones, namely, zones 602,604, and 606, labeled Zone 1, Zone 2, and Zone 3, respectively. Anintra-prediction mode of one of the zones 602, 604, 606 can be used togenerate a prediction block 607 having dimensions conforming to acurrent block. The zone 602 is characterized by intra-prediction modeshaving prediction angles greater than 0 and less than or equal to 90degrees (i.e., (0, 90] degrees). As such, a prediction mode having anangle in Zone 1 may be referred to as a north-east intra-predictionmode. The zone 604 is characterized by intra-prediction modes havingprediction angles greater than 90 and less than or equal to 180 degrees(i.e., (90, 180] degrees). As such, a prediction mode having an angle inZone 2 may be referred to as a north-west intra-prediction mode. Thezone 606 is characterized by intra-prediction modes having predictionangles between 180 and 270 degrees (i.e., (180, 270) degrees). As such,a prediction mode having an angle in Zone 3 may be referred to as asouth-west intra-prediction mode. Collectively, prediction modes ofzones 604-606 can be referred to as directional intra-prediction modes.As such, the prediction modes V_PRED, H_PRED, D45_PRED, D135_PRED,D117_PRED, D153_PRED, D207_PRED, and D63_PRED are directionalintra-prediction modes.

FIG. 6A also illustrates first pixels 608 in a row above the currentblock and second pixels 605 in a column to the left of the currentblock. The first pixels 608 and second pixels 605 can be used togenerate the prediction block. In some implementations, directionalintra-prediction modes in Zone 1 (i.e., intra-prediction modes havingprediction angles between 0 and 90 degrees) use the first pixels 608 butmay not use the second pixels 605 to generate the prediction block;directional predictions in Zone 2 (i.e., intra-prediction modes havingprediction angles between 90 and 180 degrees) use the first pixels 608and the second pixels 605 to generate the prediction block; anddirectional predictions in Zone 3 (i.e., intra-prediction modes havingprediction angles between 180 and 270 degrees) use the second pixels 605but may not use the first pixels 608 to generate the prediction block.

FIG. 6B is a diagram of examples of intra-prediction modes according toimplementations of this disclosure. In the examples of FIG. 6B, thepixels A-M can be, or can be a subset of, the first pixels 608 of FIG.6A; the pixels I-M can be, or can be a subset of, the second pixels 605of FIG. 6A.

An intra-prediction mode 610 illustrates the V_PRED intra-predictionmode, which is referred to generally as a vertical intra-predictionmode. In this mode, prediction block pixels in the first column are setto the value of peripheral pixel A; prediction block pixels in thesecond column are set to the value of pixel B; prediction block pixelsin the third column are set to the value of pixel C; and predictionblock pixels in the fourth column are set to the value of pixel D.

An intra-prediction mode 620 illustrates the H_PRED intra-predictionmode, which is referred to generally as a horizontal intra-predictionmode. In this mode, prediction block pixels in the first row are set tothe value of peripheral pixel I; prediction block pixels in the secondrow are set to the value of pixel J; prediction block pixels in thethird row are set to the value of pixel K; and prediction block pixelsin the fourth row are set to the value of pixel L.

An intra-prediction mode 630 illustrates the D117_PRED intra-predictionmode, so called because the direction of the arrows, along which theperipheral pixels will be propagated to generate the prediction blockform a diagonal, is at an angle of about 117° from the horizontal. Thatis, in the D117_PRED, the prediction angle is 117°. The intra-predictionmode 630 is a Zone 2 (i.e., a north-west) intra-prediction mode.

An intra-prediction mode 640 illustrates the D63_PRED intra-predictionmode, which corresponds to a prediction angle of 63°. Theintra-prediction mode 640 is a Zone 3 (i.e., a south-west)intra-prediction mode. An intra-prediction mode 650 illustrates theD153_PRED intra-prediction mode, which corresponds to a prediction angleof 153°. The intra-prediction mode 650 is a Zone 2 (i.e., a north-west)intra-prediction mode. An intra-prediction mode 660 illustrates theD135_PRED intra-prediction mode, which corresponds to a prediction angleof 135°. The intra-prediction mode 660 is a Zone 1 (i.e., a north-east)intra-prediction mode.

The prediction modes D45_PRED and D207_PRED (not shown) correspond,respectively, to the prediction angles 45° (Zone 1 or north-eastintra-prediction mode) and 207° (Zone 3 or south-west intra-predictionmode). The DC_PRED corresponds to a prediction mode where all predictionblock pixels are set to a single value that is a combination of theperipheral pixels A-M.

The PAETH_PRED intra-prediction mode is an example of a non-directionalintra-prediction mode. In the PAETH_PRED intra-prediction mode, theprediction value of a pixel is determined as follows: 1) calculate abase value as a combination of some peripheral pixels, and 2) use, asthe prediction pixel, the one of the some peripheral pixels that isclosest to the base value. The PAETH_PRED intra-prediction mode isillustrated using, as an example, a pixel 612 (at location x=1, y=2). Inan example of a combination of some peripheral pixels, the base valuecan be calculated as base=B+K−M. That is, the base value is equal to:the value of the left peripheral pixel that is in the same row as thepixel to be predicted+the value of the above peripheral pixel that is inthe same column as the pixel−the value of the pixel in the top-leftcorner.

FIG. 7 is a diagram of examples of smooth intra-prediction modesaccording to implementations of this disclosure. In FIG. 7, anintra-prediction mode 700 illustrates the SMOOTH_V_PRED intra-predictionmode, an intra-prediction mode 730 illustrates the SMOOTH_H_PREDintra-prediction mode, and an intra-prediction mode 760 illustrates theSMOOTH_PRED intra-prediction mode.

The description herein refers to “neighboring pixels” (e.g., “firstneighboring pixels” and “second neighboring pixels”) and “pixels” (e.g.,“second pixels” and “fourth pixels”). A “pixel” is understood to be at alocation and to have a value. For simplicity, instead of, for example,using “neighboring pixels at a location” and “neighboring pixel values,”only “neighboring pixel” may be used. The context should make clearwhether a location or a value is being referred to. Additionally, a“pixel value” can also mean a luminance value (i.e., a Y value) or achrominance value (e.g., a U or a V value). As such, selecting,generating, calculating, or the like, a pixel value encompassesselecting, generating, or calculating a luminance pixel value;selecting, generating, or calculating a chrominance U pixel value; orselecting, generating, or calculating a chrominance V pixel value.

The SMOOTH_V_PRED intra-prediction mode (or, simply, prediction mode)can be summarized as follows. Using available reconstructed pixels alonga first (e.g., top or bottom) horizontal edge of a current block,generate one or more pixel values along the opposite horizontal edge(e.g., bottom or top), and interpolate pixels of the first edge and theopposite edge to determine the pixel values of the prediction block. Theone or more pixel values along the opposite horizontal edge can begenerated using available reconstructed pixels peripheral to a vertical(e.g., left or right) edge of the current block. If reconstructed pixelsperipheral to a vertical edge are not available, such as when thecurrent block is at the edge of a frame, then a default value (e.g.,128) can be used instead of reconstructed pixels.

The SMOOTH_H_PRED intra-prediction mode can be summarized as follows.Using available reconstructed pixels along a first (e.g., right or left)vertical edge of a current block, generate one or more pixel valuesalong the opposite vertical edge (e.g., left or right), and interpolatepixels of the first edge and the opposite edge to determine the pixelvalues of the prediction block. The one or more pixel values along theopposite vertical edge can be generated using available reconstructedpixels peripheral to a horizontal (e.g., top or bottom) edge of thecurrent block. If reconstructed pixels peripheral to a horizontal edgeare not available, such as when the current block is at the edge of aframe, then a default value (e.g., 128) can be used instead ofreconstructed pixels.

The SMOOTH_PRED intra-prediction mode combines the operations of theSMOOTH_V_PRED and SMOOTH_H_PRED prediction modes. That is, for example,for a pixel of the prediction block, a first interpolation value can bedetermined as described with respect to the SMOOTH_V_PRED, a secondinterpolation value can be determined as described with respect to theSMOOTH_H_PRED, and the first interpolation value and the secondinterpolation value can be combined (e.g., averaged, added and scaled,etc.) to set the corresponding pixel of the prediction block.

As used in this disclosure, “determine” means to create, form, produce,select, construct, identify, specify, calculate, generate, or otherwisedetermine in any manner whatsoever. As used in this disclosure,“generate” means to create, form, produce, select, construct, identify,specify, calculate, determine, or otherwise generate in any mannerwhatsoever.

Illustrated with respect to the SMOOTH_V_PRED intra-prediction mode(i.e., the intra-prediction mode 700) are a prediction block 702, firstneighboring pixels 706, second pixels 708, and third neighboring pixels710. The prediction block 702 is the prediction block to be generated.The prediction block is coextensive with (i.e., has the same size as) acurrent block being predicted.

The first neighboring pixels 706 can be as described with respect to thefirst pixels 608 of FIG. 6A or the pixels A-M of FIG. 6B. As such, thefirst neighboring pixels 706 are peripheral to (e.g., adjoining andoutside of) the current block along a first edge of the block. The firstedge is illustrated as being the top edge. However, that need not be thecase: Which peripheral reconstructed pixels are available depends on thescan order of the current image that includes the current block.

The third neighboring pixels 710 can be as described with respect to thesecond pixels 605 of FIG. 6A or the pixels I-M of FIG. 6B. As such, thethird neighboring pixels 710 are peripheral to (i.e., adjoining andoutside of) the current block along a third edge of the block. The thirdedge is illustrated as being the left edge. However, that need not bethe case: Which peripheral reconstructed pixels are available depends onthe scan order of the current image that includes the current block.

At least one of the pixels of the third neighboring pixels 710 can beused to generate the second pixels 708. It is noted that the secondpixels 708 are not available pixels. That is, the second pixels 708 arenot reconstructed pixels. The second pixels 708 are calculated pixels.The second pixels 708 are calculated using pixels of the thirdneighboring pixels 710. That is, the pixels of the second pixels 708(e.g., a row) are calculated from the adjoining and reconstructed thirdneighboring pixels (e.g., a column). Much like the first neighboringpixels 706 are peripheral to the current block, the second pixels 708are also thought of as being peripheral to the current block. The secondpixels 708 are peripheral along an edge that is parallel to the edge towhich the first neighboring pixels 706 are peripheral. That is, forexample, if the first neighboring pixels 706 are peripheral to the topedge, then the second pixels 708 are thought of (e.g., considered to be)peripheral to the bottom edge.

In an example, each pixel, such as a pixel 714, of the second pixels 708is set to the value of a reconstructed pixel 716 of the thirdneighboring pixels 710. The reconstructed pixel 716 is the reconstructedpixel that is aligned with the last row of the current block (or,equivalently, the prediction block). For example, if the current block(or, equivalently, the prediction block 702) is of size M×N (e.g., 4×4,where M=4), then the reconstructed pixel 716 is the fourth reconstructedpixel of the third neighboring pixels 710. That is, the reconstructedpixel 716 can be as illustrated with respect to the pixel L of theintra-prediction modes 610-660.

In an example, the third neighboring pixels 710 can have more availablepixels than shown in FIG. 6B. For example, the second pixels 605 of FIG.6A show that more reconstructed pixels are available. As such, insteadof using the reconstructed pixel 716, a reconstructed pixel 718,adjoining the second pixels 708 (i.e., adjoining the assumed locationfor the second pixels 708), can be used to generate the values of thesecond pixels 708.

In an example, even if the reconstructed pixel 718 is available, thesecond pixels 708 can be generated using the reconstructed pixel 716. Inanother example, a combination of any of the third neighboring pixels710 can be used to set the values of the second pixels 708.

As described above, the pixels of the second pixels 708 can be set tothe pixel value of one of the reconstructed pixels 716, 718 (i.e., apixel value of BL). As such, each of the pixels of the second pixels 708can have the same value, BL. As such, the second pixels 708 can mean onepixel value (i.e., BL). In another example, each pixel of the secondpixels 708 can have a different value. For example, a value of a pixelof the second pixels 708 can be calculated based on the BL value and thedistance of the pixel from the edge.

The first neighboring pixels 706 and the second pixels 708 can be usedto determine the pixel values of the prediction block 702. Each pixel704 of the prediction block 702 can be calculated as an interpolation ofa corresponding pixel of the first neighboring pixels 706 and acorresponding pixel of the second pixels 708. It is noted that since onevalue (i.e., BL) can be used for all pixels of the second pixels 708,the second pixels 708 can be only one pixel (i.e., one pixel value).

Each pixel P_(i,j) (i.e., a pixel in row i) of the prediction block 702can be calculated as a scaled weighted sum of a pixel T_(i) (e.g., apixel 712 of the first neighboring pixels 706) that is in the samecolumn j as the pixel P_(i,j) and a corresponding pixel of the secondpixels 708 (e.g., the value BL). The weights can depend on the distanceof the pixel P_(i,j) from the top and bottom edges of the predictionblock 702.

In an example, a linear interpolation is used. As such, the weights ofthe interpolation, which may be values between 0 and 1, can be evenlydistributed. That is, for example, given a block of size M×N, where M isthe number of rows, then the weights can be proportional to M. That is,a row X is assigned a weight (X+1)/M, where X=0, 1, . . . , N−1. Forexample, for a 4×4 block, where M=4, the weights are 0.25, 0.5, 0.75,and 1. However, other weights can be used.

Other interpolation techniques can also be used. For example, theinterpolation can be a cosine of 0 to 90 degrees interpolation, a cosineof 0 to 180 degrees interpolation, an exponential interpolation, a cubicinterpolation, a quadratic below linear, a quadratic above linear, or aquadratic interpolation.

In an example, a quadratic interpolation (in this case, a quadraticinterpolation in the vertical direction) can be used. For example, thesquare of the interpolation weights can be distributed between 0 and 1.

To summarize, the SMOOTH_V_PRED mode can work as follows:

-   -   1) Estimate the bottom row pixels to be the same as a        combination of the left reconstructed pixels (the combination of        the left reconstructed pixels can simply be the bottom-left        pixel (BL)); and    -   2) Estimate the pixels (P_(i,j)) within the prediction block        using an interpolation (e.g., a quadratic interpolation in the        vertical direction) between the top row and the bottom row        (i.e., the estimated or assumed row).

The following VerticalSmoothPredictorBlock function is an example of animplementation that can be used by a codec to generate the predictionblock using the quadratic interpolation in the vertical direction.

function VerticalSmoothPredictorBlock(rows, cols, left_col, top_row,top_left, pred) 1. below_pred = left_col[rows − 1] 2. vert_weights =GetWeights(rows) 3. sm_weight_log2_scale = 8 4. scale = (1 <<sm_weight_log2_scale); 5. for row in 0 to rows − 1, repeat: 6.  for colin 0 to cols − 1, repeat: 7.   total = top_row[col] *vert_weights[row] +         below_pred * (scale − vert_weights[row]) 8.   pred[row][col] = Clip(total / scale)

The function VerticalSmoothPredictorBlock receives, as input, rows(i.e., the number of rows in the prediction block), cols (i.e., thenumber of columns in the prediction block), left_col (i.e., an array ofthe left reconstructed pixels, such as the third neighboring pixels 710of FIG. 7 or the second pixels 605 of FIG. 6A), top row (i.e., an arrayof the top reconstructed pixels, such as the first pixels 608 of FIG. 6or the first neighboring pixels 706 of FIG. 7), and top_left (i.e., thetop-left pixel reconstructed pixel value, such as the pixel M of FIG.6). The function VerticalSmoothPredictorBlock generates, as output, pred(i.e., the prediction block to be generated).

At operation 1, the value below_pred (i.e., the second pixels 708) isset to the last value of the left neighboring reconstructed pixels(i.e., the reconstructed pixel 716). However, as indicated above, thebelow pred can be set to any combination of the third neighboring pixels710. Also, as indicated, the below pred can be set (e.g., an array) tothe reconstructed pixel 718. The variable vert_weights is assigned, atoperation 2, to a pointer into a weights lookup table. The weights areused for the quadratic interpolation. The interpolation weights arelooked up in a weights table (via a call to the function GetWeights), asfurther described with respect to FIG. 8.

Interpolation coefficients (i.e., weights) are typically values between0 and 1. However, as performing integer arithmetic is more efficient,the weights are scaled to integer values. A scaling factor of 8 is usedto correspond to the pixel values to be generated for the predictionblock. In the example of the above pseudo-code, generated pixel valuesare to be values between 0 and 255 (i.e., 2⁸−1), as such a variable,sm_weight_log 2_scale, to be used as a scaling factor, is set to 8 atoperation 3. Correspondingly, a scaling factor, scale, is used as themaximum interpolation weight. The value of scale is set at operation 4to “1 left-shifted by 8 bits,” providing a value of 256. Similarly, theresult of the interpolation is scaled, at operation 8, to a valuebetween 0 and 255 using the variable scale—the function Clip( ) clips avalue to a range of 0 and 255 (both inclusive).

For each of the prediction pixels of the prediction block pred, such asa pixel at a row row and column col, a total value is calculated atoperation 7. The total value corresponds to the weighted sum of thereconstructed pixel in the same column col (i.e., top_row[col]) and thebelow pred value (i.e., the value BL, as described above).

FIG. 8 is an example of weights used for a quadratic interpolationaccording to implementations of this disclosure. FIG. 8 includes aweights table 810, sm_weights_arrays. The weights table 810 isillustrated as a one-dimensional array; however, the weights table canbe implemented using other data structures.

As described with respect to FIG. 7, the GetWeights function is used toreturn an index into the weights table 810. The GetWeights functionreceives, as input, a block size, and returns the location (i.e., apointer to the location) of the weights to be used with each row of theblock. The following is an example of an implementation of theGetWeights function:

-   -   function GetWeights(dim)    -   1. return sm_weight_arrays+dim

The weights table 810 is shown to include weights for blocks of amaximum of 64 rows or columns. However, the disclosure is not solimited. The weights table 810 can include weights for largerdimensions.

In an example of the operation of the GetWeights function, if a blockdimension of 4 is input to the GetWeights function, then the GetWeightsfunction returns a pointer to (i.e., the index of) a weight 812. Assuch, the weights 255, 149, 85, and 64 are used, respectively, atoperation 8 of the VerticalSmoothPredictorBlock function, for the valuesof the iteration variable col, namely, col=0, 1, 2, and 3. Similarly, ifa block dimension of 64 is input to the GetWeights function, then theGetWeights function returns a pointer to (i.e., the index of) a weight814.

In an implementation, instead of looking up the weights in the weightstable 810, the weights can be calculated when required. A program 850 isan example of generating at least part of the weights table 810. Assuch, instead of pre-generating the weights table 810 and using theweights table 810 as a lookup table, a person skilled in the art canappreciate that the program 850, or similar programs and functions, canbe used instead of the weights table 810.

Illustrated with respect to the SMOOTH_H_PRED intra-prediction mode(i.e., the intra-prediction mode 730) are a prediction block 732, firstneighboring pixels 736, second pixels 738, and third neighboring pixels740. The prediction block 732 is the prediction block to be generated.The prediction block is coextensive with (i.e., has the same size as) acurrent block being predicted.

The first neighboring pixels 736 can be as described with respect to thesecond pixels 605 of FIG. 6A or the pixels I-M of FIG. 6B. As such, thefirst neighboring pixels 736 are peripheral to (e.g., adjoining andoutside of) the current block along a first edge of the block. The firstedge is illustrated as being the left edge. However, that need not bethe case: Which peripheral reconstructed pixels are available depends onthe scan order of the current image that includes the current block.

The third neighboring pixels 740 can be as described with respect to thefirst pixels 608 of FIG. 6A or the pixels A-H of FIG. 6B. As such, thethird neighboring pixels 740 are peripheral to (i.e., adjoining andoutside of) the current block along a third edge of the block. The thirdedge is illustrated as being the top edge. However, that need not be thecase: Which peripheral reconstructed pixels are available depends on thescan order of the current image that includes the current block.

At least one of the pixels of the third neighboring pixels 740 can beused to generate the second pixels 738. It is noted that the secondpixels 738 are not available pixels. That is, the second pixels 738 arenot reconstructed pixels. The second pixels 708 are calculated pixels.The second pixels 738 are calculated using pixels of the thirdneighboring pixels 740. That is, the pixels of the second pixels 738(e.g., a column) are calculated from the adjoining and reconstructedthird neighboring pixels (e.g., a row). Much like the first neighboringpixels 736 are peripheral to the current block, the second pixels 738are also thought of as being peripheral to the current block. The secondpixels 738 are peripheral along an edge that is parallel to the edge towhich the first neighboring pixels 736 are peripheral. That is, forexample, if the first neighboring pixels 736 are peripheral to the leftedge, then the second pixels 738 are thought of (e.g., considered to be)peripheral to the right edge.

In an example, each pixel, such as a pixel 744, of the second pixels 738is set to the value of a reconstructed pixel 746 of the thirdneighboring pixels 740. The reconstructed pixel 746 is the reconstructedpixel that is aligned with the last column of the current block (or,equivalently, the prediction block). For example, if the current block(or, equivalently, the prediction block 732) is of size M×N (e.g., 4×4,where N=4), then the reconstructed pixel 746 is the fourth reconstructedpixel of the third neighboring pixels 740. That is, the reconstructedpixel 746 can be as illustrated with respect to the pixel D of theintra-prediction modes 610-660.

In an example, instead of using the reconstructed pixel 746, areconstructed pixel 748, adjoining the second pixels 738 (i.e.,adjoining the assumed location for the second pixels 738), can be usedto generate the values of the second pixels 738.

As described above, the pixels of the second pixels 738 can be set tothe pixel value of one of the reconstructed pixels 746, 748 (i.e., apixel value of TR). As such, each of the pixels of the second pixels 738can have the same value, TR. As such, the second pixels 738 can mean onepixel value (i.e., RL).

The first neighboring pixels 736 and the second pixels 738 can be usedto determine the pixel values of the prediction block 732. Each pixel734 of the prediction block 732 can be calculated as an interpolation ofa corresponding pixel of the first neighboring pixels 736 and acorresponding pixel of the second pixels 738. It is noted that in thecase where one value (i.e., TR) is used for all pixels of the secondpixels 738, the second pixels 738 can be only one pixel (i.e., one pixelvalue).

Each pixel P_(i,j) (i.e., a pixel in column j) of the prediction block732 can be calculated as a scaled weighted sum (i.e., an interpolation)of a pixel L_(j) (e.g., a pixel 742 of the first neighboring pixels 736)that is in the same row as the pixel P_(i,j) and a corresponding pixelof the second pixels 738 (e.g., the value TR). The weights can depend onthe distance of the pixel P_(i,j) from the left and right edges of theprediction block 732.

To summarize, the SMOOTH_H_PRED mode can work as follows:

-   -   1) Estimate the right column pixels to be the same as a        combination of the top reconstructed pixels (the combination can        simply be the top-right pixel (TR)); and    -   2) Estimate the pixels (Pi,j) within the prediction block using        an interpolation (e.g., a quadratic interpolation in the        horizontal direction) between the left column and the right        column (i.e., the estimated row).

The SMOOTH_H_PRED intra-prediction mode can use one of the interpolationtechniques described with respect to the SMOOTH_V_PRED described above.

In an example, the following HorizontalSmoothPredictorBlock function isan example of an implementation that can be used by a codec to generatethe prediction block using a quadratic interpolation in the horizontaldirection. The function HorizontalSmoothPredictorBlock is similar inmany respects to the function VerticalSmoothPredictorBlock describedabove. As such, a description of the functionHorizontalSmoothPredictorBlock is omitted.

function HorizontalSmoothPredictorBlock(rows, cols, left_col, top_row,top_left, pred) 1. right_pred = top[cols − 1] 2. hor_weights =GetWeights(cols) 3. sm_weight_log2_scale = 8 4. scale = (1 <<sm_weight_log2_scale); 5. for row in 0 to rows − 1, repeat: 6.   for colin 0 to cols − 1, repeat: 7.      total = left[row] * hor_weights[col] +           right_pred * (scale − hor_weights[col])8.      pred[row][col] = Clip(total / scale)

Illustrated with respect to the SMOOTH_PRED intra-prediction mode (i.e.,the intra-prediction mode 760) are a prediction block 762 and aprediction pixel 764. The intra-prediction mode 760 also includes thepixel 712, the pixel 714, and the reconstructed pixels 716, which are asdescribed with respect to the intra-prediction mode 700; and the pixel742, the pixel 744, and reconstructed pixels 746, which are as describedwith respect to the intra-prediction mode 730.

SMOOTH_PRED combines the operations of the SMOOTH_V_PRED and theSMOOTH_H_PRED intra-prediction modes. That is, for example, for theprediction pixel 764 of the prediction block, a first interpolationvalue is determined using the pixel 742 and the pixel 744, a secondinterpolation value is determined using the pixel 712 and the pixel 714,and the first interpolation value and the second interpolation value canbe combined (e.g., averaged, scaled, etc.).

To summarize, the SMOOTH_PRED mode can work as follows:

-   -   1. Perform a vertical interpolation by:        -   a. Estimating the bottom row pixels to be the same as a            bottom-left pixel (e.g., BL); and        -   b. Calculating a vertical interpolation value (Vi,j) within            the block using a vertical interpolation (e.g., a quadratic            interpolation in the vertical direction) between the top row            and the bottom row.    -   2. Perform a horizontal interpolation by:        -   a. Estimating the right column pixels to be the same as a            top-right pixel (e.g., TR); and        -   b. Calculating a horizontal interpolation value (Hi,j)            within the block using a horizontal interpolation (e.g., a            quadratic interpolation in the horizontal direction) between            the left row and the right column.    -   3. Calculate final prediction pixel values by:        -   a. Calculating each pixel Pi,j as the average of Vi,j and            Hi,j.

The following SmoothPredictorBlock function is an example of animplementation that can be used by a codec to generate the predictionblock using the quadratic interpolation in both the vertical directionand the horizontal direction. The function SmoothPredictorBlock issimilar to the VerticalSmoothPredictorBlock andHorizontalSmoothPredictorBlock functions. As such, a description of thefunction SmoothPredictorBlock is omitted.

function SmoothPredictorBlock(rows, cols, left_col, top_row, top_left,pred)  1. below_pred = left[rows − 1]  2. right_pred = top[cols − 1]  3.hor_weights = GetWeights(cols)  4. vert_weights = GetWeights(rows)  5.sm_weight_log2_scale = 8  6. scale = (1 << sm_weight_log2_scale);  7.for row in 0 to rows−1, repeat:  8.   for col in 0 to cols − 1, repeat: 9.   total = above[col]*vert_weights[row] +           below_pred*(scale − vert_weights[row]) +         left[row] *hor_weights[col] +            right_pred * (scale − hor_weights[col])10.      pred[row][col] = Clip(total / (scale*2))

At operation 9, each of the sub-expressions “above [col] *vert_weights[row]+below_pred*(scale−vert_weights[row])” and “left[row]*hor_weights[col]+right_pred*(scale−hor_weights [col]) ” produces avalue that is in the scale of scale (e.g., 256). As such, the total isin the scale of scale*2 (e.g., 512). As such, at operation 10, the totalvalue is divided by (scale*2).

The intra-prediction mode selected by the encoder can be transmitted toa decoder in the bitstream. The intra-prediction mode can be entropycoded (encoded by the encoder and/or decoded by a decoder) using acontext model.

Some codecs use the intra-prediction modes of the left and aboveneighbor blocks as the context for coding the intra-prediction mode of acurrent block. Using FIG. 6 as an example, the left neighbor block canbe the block containing the pixels I-L, and the above neighbor block canbe the block containing the pixels A-D.

A codec can include a probability distribution for each combination ofintra-prediction modes used by the left neighbor block and aboveneighbor block. As such, using the above 13 prediction modes, 169probability distributions (corresponding to 13*13=172 contexts) may bestored. To retrieve a probability distribution, the codec can perform aprocedure that includes the operations:

cdf_prob kf_y_mode_cdf[13][13][13]; left = left neighbor intra mode (orDC_PRED if unavailable) above = above neighbor intra mode (or DC_PRED ifunavailable) prob_table = kf_y_mode_cdf[left][above];

For ease of reference, the above procedure is referred to as thectx-combinations technique. In the ctx-combinations technique, leftstores the intra-prediction mode of the left neighbor block. If theintra-prediction mode of the left neighbor block is not available, thenthe DC_PRED intra-prediction mode may be assumed. above stores theintra-prediction mode of the top neighbor block. If the intra-predictionmode of the above neighbor block is not available, then the DC_PREDintra-prediction mode may be assumed.

An intra-prediction mode may not be available because, for example, thecurrent block is at the edge (e.g., is the topmost and/or leftmostblock) of the frame or because the neighbor block (left or above) wasinter-predicted. The intra-prediction mode may not be available forother reasons. The probability distribution (i.e., prob_table) can beretrieved from a three-dimensional array (i.e., kf_y_mode_cdf) thatincludes the probability distributions for all combinations of left andabove intra-prediction modes. In an example, the first dimensioncorresponds to the left neighbor intra-prediction mode (e.g., 13values), the second dimension corresponds to the above neighborintra-prediction mode (e.g., 13 values), and the third dimensioncorresponds to the values of a probability distribution.

As indicated above, each probability distribution includes 13probability values (i.e., a probability value for each of the predictionmodes). As such, there are 12 freedoms (i.e., only 12 of the 13probability values are stored per context). In some implementations, a13th entry that is a constant (e.g., a 0) may also be stored.

The number of stored probability distributions and/or probability valuesper probability distribution can be reduced by reducing the number ofcontexts for coding an intra-prediction mode.

In an example, the number of contexts can be reduced when some of theintra-prediction modes exhibit certain characteristics. For example, theprobabilities of some of the intra-prediction modes may notsignificantly change based on the left intra-prediction mode and/or theabove intra-prediction mode. Such intra-prediction modes may be equallyprobable regardless of the left and above intra-prediction modes. In anexample, the SMOOTH_PRED, SMOOTH_V_PRED, and SMOOTH_H_PRED predictionmodes may exhibit such characteristics.

Implementations according to this disclosure can reduce the number ofcontexts available for coding the intra-prediction mode of a currentblock using a procedure that includes the operations:

1. cdf_prob kf_y_mode_cdf[13][2][13]; 2. left = left neighbor intra mode(or DC_PRED if unavailable) 3. above = above neighbor intra mode (orDC_PRED if unavailable) 4. if (left == above)   a. prob_table =kf_y_mode_cdf[left][0]; 5. else if (left != SMOOTH_PRED && left !=SMOOTH_H_PRED &&     left != SMOOTH_V_PRED)   a. prob_table =kf_y_mode_cdf[left][1]; 6. else   a. prob_table =kf_y_mode_cdf[above][1];

For ease of reference, the above procedure is referred to as thefirst-ctx-reduction technique. Using the first-ctx-reduction technique,the number of contexts can be reduced to 26. As such, the number ofcontexts used to code (encode/decode) the intra-prediction modes ofluminance blocks can be reduced by approximately 85%. In the operations4-4a above, if the left neighbor block and the above neighbor block usethe same intra-prediction mode, then the left intra-prediction mode isused to retrieve a probability distribution. This accounts for 13probability distributions. The operations 5-6 can be summarized as: Ifone of the left or above neighbors uses a smooth prediction mode (i.e.,one of SMOOTH_PRED, SMOOTH_V_PRED, or SMOOTH_H_PRED), then retrieve aprobability distribution using the other intra-prediction mode. Theseoperations account for another 13 probability distributions.

In an implementation, the operations can be used to determine thecontext for coding the intra-prediction modes of blocks of a key frame(also known as a golden frame). In a key frame, all blocks are predictedusing intra-prediction.

FIG. 9 is a flowchart diagram of a process 900 for intra-coding acurrent block according to an implementation of this disclosure. Theprocess 900 can be implemented by an encoder, such as the encoder 400 ofFIG. 4. When implemented by an encoder, “coding” means encoding in anencoded bitstream, such as the compressed bitstream 420 of FIG. 4. Forexample, the process 900 can be performed in whole or in part by theentropy encoding stage 408 of the encoder 400. The process 900 can beperformed by a decoder, such as the decoder 500 of FIG. 5. Whenimplemented by a decoder, “coding” means decoding from an encodedbitstream, such as the compressed bitstream 420 of FIG. 5. For example,the process 900 can be performed in whole or in part by the entropydecoding stage 502 of the decoder 500, and the encoded video bitstreamcan be the compressed bitstream 420 of FIG. 5.

Implementations of the process 900 can be performed by storinginstructions in a memory, such as the memory 204 of the receivingstation 106, to be executed by a processor, such as the CPU 202, forexample.

The process 900 can be implemented using specialized hardware orfirmware. Some computing devices can have multiple memories, multipleprocessors, or both. The steps or operations of the process 900 can bedistributed using different processors, memories, or both. Forsimplicity of explanation, the process 900 is depicted and described asa series of steps or operations. However, the teachings in accordancewith this disclosure can occur in various orders and/or concurrently.Additionally, operations in accordance with this disclosure may occurwith other operations not presented and described herein. Furthermore,not all illustrated steps or operations may be used to implement amethod in accordance with the disclosed subject matter.

At 902, the process 900 codes the current block using anintra-prediction mode. At 904, the process 900 determines a leftintra-mode (i.e., an intra-prediction mode) of a left neighbor block. Inan implementation, if the left intra-mode is not available, then theleft intra-mode can be assumed to be a default left intra-predictionmode. The default left intra-prediction mode can be one of the availableintra-prediction modes. For example, the default left intra-predictionmode can be the DC_PRED mode.

At 906, the process 900 determines an above intra-mode (i.e., anintra-prediction mode) of an above neighbor block. In an implementation,if the above intra-mode is not available, then the above intra-mode canbe assumed to be a default above intra-prediction mode. The defaultabove intra-prediction mode can be one of the available intra-predictionmodes. For example, the default above intra-prediction mode can be theDC_PRED mode.

At 908, if the left intra-mode and the above intra-mode are the same,then the process 900 proceeds to 910; otherwise, the process 900proceeds to 914. At 910, the process 900 uses one of the left intra-modeor the above intra-mode (which are the same) to determine a probabilitydistribution for coding the intra-prediction mode. As such, on conditionthat the left intra-mode and the above intra-mode are equal to a samemode, the process 900 uses that same mode to determine a probabilitydistribution for coding the intra-prediction mode. As used in thisdisclosure, “determine” means to select, construct, identify, specify,generate, or otherwise determine in any manner whatsoever. For example,the probability distribution can be retrieved from a memory.

Some of the available intra-prediction modes can be classified as smoothintra-prediction modes. In an implementation, the smoothintra-prediction mode can be one of SMOOTH_PRED, SMOOTH_H_PRED, andSMOOTH_V_PRED. On condition that at least one of the left intra-mode orthe above intra-mode is the smooth intra-prediction mode, the process900 uses the other of the left intra-mode and the above intra-mode todetermine the probability distribution for coding the intra-predictionmode. Accordingly, at 914, the process 900 determines if one, but notboth, of the left intra-mode and the above intra-mode is the smoothintra-prediction mode. If so, the process 900 proceeds to 916 and usesthe one of the left intra-mode and the above intra-mode that isn't thesmooth intra-prediction mode to determine the probability distributionfor coding the intra-prediction mode; otherwise, the process 900proceeds to 918 to use either one of the left intra-mode or the aboveintra-mode to determine the probability distribution. In animplementation, the left intra-prediction mode is used at 918. From 916or 918, the process 900 proceeds to 912. At 912, the process 900 codesthe intra-prediction mode using the probability distribution.

The number of contexts for coding the intra-prediction mode of a currentblock can be reduced by splitting the available intra-prediction modesinto classes. Each class can include one or more intra-prediction modes.The number of symbols coded for an intra-prediction mode can depend onthe class of the intra-prediction mode.

In an example, the 13 intra-prediction modes described above can besplit into classes as shown in Table II.

TABLE II Intra-Prediction Mode Intra-Prediction Class DC_PRED DC H_PREDH V_PRED V SMOOTH_PRED, SMOOTH_V_PRED, SMOOTH and SMOOTH_H_PREDPAETH_PRED PAETH D45_PRED, D135_PRED, D117_PRED, DIRECTIONAL D153_PRED,D207_PRED, D63_PRED

A first symbol can indicate the intra-prediction class of theintra-prediction mode. As there are six (6) intra-prediction classes inTable II, using the ctx-combinations technique for determining aprobability distribution for coding the first symbol, a total of 36contexts are possible. The 36 contexts correspond to six (6) possibleclasses for the left neighbor block and six (6) possible classes for theabove neighbor block. Each of the contexts includes five (5) probabilityvalues. Using the first-ctx-reduction technique, only 12 (=6*2) contextsare required to code the first symbol.

If the intra-prediction mode is in the class SMOOTH, then, using thectx-combinations technique for determining a probability distributionfor coding a second symbol, a total of nine (9) contexts are possible.The nine (9) contexts correspond to three (3) possible smoothintra-prediction modes for the left neighbor block and three (3)possible smooth intra-prediction modes for the above neighbor block.Using the first-ctx-reduction technique, only one (1) context isrequired. Each context includes two (2) probability values. The SMOOTHclass can include the intra-prediction modes that are smoothintra-prediction modes, as described above. Said another way, each ofthe smooth intra-prediction modes can be mapped to the SMOOTH class.

If the intra-prediction mode is in the class DIRECTIONAL, then, usingthe ctx-combinations technique for determining a probabilitydistribution for coding the second symbol, a total of 36 contexts arepossible. The 36 contexts correspond to six (6) possibleintra-prediction modes in the DIRECTIONAL class for the left neighborblock and six (6) possible intra-prediction modes in the DIRECTIONALclass for the above neighbor block. Using the first-ctx-reductiontechnique, only 12 contexts are required. Each context provides five (5)probability values. The DIRECTIONAL class can include theintra-prediction modes that are directional intra-prediction modes, asdescribed above. Said another way, each of the directionalintra-prediction modes can be mapped to the DIRECTIONAL class.

FIG. 10 is a flowchart diagram of a process 1000 for coding a currentblock using an intra-prediction mode according to an implementation ofthis disclosure. In coding the current block using intra-prediction, anencoder can encode the intra-prediction mode used to encode the currentblock into an encoded bitstream (such as the compressed bitstream 420 ofFIG. 4). In decoding the current block using intra-prediction, a decodercan decode the intra-prediction mode to be used to decode the currentblock from an encoded bitstream (such as the compressed bitstream 420 ofFIG. 5).

The process 1000 can code one (and in some cases more than one) symbolin/from the compressed bitstream indicating the intra-prediction mode.The process 1000 codes a first symbol using a context model. The contextused (for the selection of the context model) can be, or can include, asdescribed above, the intra-prediction modes of neighboring coded blocksof the current block. The process 1000 maps the intra-prediction modesof the neighboring blocks to classes, wherein the number of classes issmaller than the number of available intra-prediction modes. The process1000 uses the classes of the intra-prediction modes of the neighboringblocks, instead of the intra-prediction modes themselves, to select thecontext model. Implementations of the process 1000 can reduce the numberof contexts used for coding the intra-prediction mode.

The process 1000 can be performed by a decoder, such as the decoder 500of FIG. 5. For example, the process 1000 can be performed in whole or inpart by the entropy decoding stage 502 of the decoder 500, and theencoded video bitstream can be the compressed bitstream 420 of FIG. 5.The process 1000 can be implemented by an encoder, such as the encoder400 of FIG. 4. For example, the process 1000 can be performed in wholeor in part by the entropy encoding stage 408 of the decoder 500.Implementations of the process 1000 can be performed by storinginstructions in a memory, such as the memory 204 of the receivingstation 106, to be executed by a processor, such as the CPU 202, forexample. The process 1000 can be implemented using specialized hardwareor firmware. Some computing devices can have multiple memories, multipleprocessors, or both. The steps or operations of the process 1000 can bedistributed using different processors, memories, or both. Forsimplicity of explanation, the process 1000 is depicted and described asa series of steps or operations.

At 1002, the process 1000 determines a first intra-prediction class of afirst intra-prediction mode used for decoding a first neighboring blockof the current block. At 1004, the process 1000 determines a secondintra-prediction class of a second intra-prediction mode used fordecoding a second neighboring block of the current block. The firstneighboring block and the second neighboring block can be, respectively,the above and left neighboring blocks of the current block, or viceversa. Referring to FIG. 6B as an example, and where the framecontaining the current block is processed in a raster scan order, theabove block can be the block that contains some of the pixels A-H, andthe left block can be the block that contains some of the pixels I-L.

The first intra-prediction mode and the second intra-prediction mode areselected from a list of available intra-prediction modes. In animplementation, the first intra-prediction mode and the secondintra-prediction mode are each selected from a set that includes theabove-described DC_PRED, V_PRED, H_PRED, D45_PRED, D135_PRED, D117_PRED,D153_PRED, D207_PRED, D63_PRED, SMOOTH_PRED, SMOOTH_V_PRED,SMOOTH_H_PRED, and PAETH_PRED intra-prediction modes. More, fewer, orother intra-prediction modes may be available.

The process 1000 can determine the first intra-prediction class and thesecond intra-prediction class using a mapping that maps intra-predictionmodes to classes. In an implementation, a mapping, such as the onedescribed with respect to Table II, can be used. As such, the firstintra-prediction class and the second intra-prediction class are eachselected from a set that includes the above-described DC class,horizontal class (i.e., H class), vertical class (i.e., V class), asmooth class (i.e., SMOOTH class), a paeth class (i.e., PAETH class),and a directional class (i.e., DIRECTIONAL class). However, othermappings may be available.

Table II maps the 13 available intra-prediction modes to six classes.The directional class can include the D45_PRED, D135_PRED, D117_PRED,D153_PRED, D207_PRED, and D63_PRED intra-prediction modes. The smoothclass can include the SMOOTH_PRED, SMOOTH_V_PRED, and SMOOTH_H_PREDintra-prediction modes.

At 1006, the process 1000 codes the intra-prediction mode using thefirst intra-prediction class and the second intra-prediction class.

In an implementation, coding the intra-prediction mode using the firstintra-prediction class and the second intra-prediction class can includecoding a first symbol indicative of a class of the intra-prediction modeand, on condition that the intra-prediction mode is one of a smoothintra-prediction mode or a directional intra-prediction mode, coding asecond symbol indicative of the intra-prediction mode.

The first symbol can be a symbol of an alphabet, such that each of thesymbols of the alphabet is indicative of one of the intra-predictionclasses. As such, one symbol is indicative of the DC class, a secondsymbol is indicative of the horizontal class, a third symbol isindicative of the vertical class, and so on.

When implemented by an encoder, the process 1000 can determine that theintra-prediction mode used to code the current block is a smoothintra-prediction mode by determining that the intra-prediction mode isone of the SMOOTH_PRED, SMOOTH_V_PRED, and SMOOTH_H_PREDintra-prediction modes. Alternatively, the process 1000 can determinethat the intra-prediction mode is a smooth intra-prediction mode if theclass of the intra-prediction mode is the smooth class. Similarly, theprocess 1000 can determine that the intra-prediction mode is adirectional intra-prediction mode by determining that theintra-prediction mode is one of the D45_PRED, D135_PRED, D 117_PRED,D153_PRED, D207_PRED, and D63_PRED intra-prediction modes.Alternatively, the process 1000 can determine that the intra-predictionmode is a directional intra-prediction mode if the class of theintra-prediction mode is the directional class. Alternatively, theprocess 1000 can determine that the intra-prediction mode is one of asmooth intra-prediction mode or a directional intra-prediction mode byusing the first symbol.

When implemented by a decoder, the process 1000 can determine that theintra-prediction mode is one of a smooth intra-prediction mode or adirectional intra-prediction mode by using the first symbol. That is,after decoding the first symbol, the process 1000 can use the firstsymbol to determine whether the intra-prediction mode is one of a smoothintra-prediction mode or a directional intra-prediction mode.

In an implementation, only one context model is available for coding anyof the smooth intra-prediction modes. As such, if the intra-predictionmode is a smooth intra-prediction mode, the process 1000 selects thesingle available context model for coding the second symbol. Whenimplemented by a decoder, the process 1000 can determine that theintra-prediction mode is a smooth intra-prediction mode by using thefirst symbol to determine the class of the intra-prediction mode. Inanother implementation, more than one context model can be available forcoding a smooth intra-prediction mode. As such, the process 1000 codesanother symbol in order to select a context model.

In an implementation, only two context models are available for coding adirectional intra-prediction mode: a first context model and a secondcontext model. As such, coding the second symbol indicative of theintra-prediction mode can include a condition that when theintra-prediction mode is a directional intra-prediction mode, if thefirst intra-prediction class and the second intra-prediction class are asame class (i.e., are equal), the process 1000 selects the first contextmodel for coding the second symbol; and if the first intra-predictionclass and the second intra-prediction class are different classes (i.e.,are not equal), the process 1000 selects the second context model forcoding the second symbol.

FIG. 11 is a flowchart diagram of a process 1100 for coding a block of avideo frame using an intra-prediction mode according to animplementation of this disclosure. The block of the video frame can be ablock of a still image. The intra-prediction mode can be one of theSMOOTH_PRED, the SMOOTH_H_PRED, or the SMOOTH_V_PRED intra-predictionmodes described above.

The process 1100 can be implemented by an encoder, such as the encoder400 of FIG. 4. When implemented by an encoder, “coding” means encodingin an encoded bitstream, such as the compressed bitstream 420 of FIG. 4.For example, the process 1100 can be performed in whole or in part bythe intra/inter-prediction stage 402 of the encoder 400. The process1100 can be performed by a decoder, such as the decoder 500 of FIG. 5.When implemented by a decoder, “coding” means decoding from an encodedbitstream, such as the compressed bitstream 420 of FIG. 5. For example,the process 1100 can be performed in whole or in part by theintra/inter-prediction stage 508 of the decoder 500, and the encodedvideo bitstream can be the compressed bitstream 420 of FIG. 5.

Implementations of the process 1100 can be performed by storinginstructions in a memory, such as the memory 204 of the receivingstation 106, to be executed by a processor, such as the CPU 202, forexample.

The process 1100 can be implemented using specialized hardware orfirmware. Some computing devices can have multiple memories, multipleprocessors, or both. The steps or operations of the process 1100 can bedistributed using different processors, memories, or both. Forsimplicity of explanation, the process 1100 is depicted and described asa series of steps or operations. However, the teachings in accordancewith this disclosure can occur in various orders and/or concurrently.Additionally, operations in accordance with this disclosure may occurwith other operations not presented and described herein. Furthermore,not all illustrated steps or operations may be used to implement amethod in accordance with the disclosed subject matter.

At 1102, the process 1100 selects first neighboring pixels. The firstneighboring pixels are peripheral to the block along a first edge of theblock. For example, when the intra-prediction mode is the SMOOTH_V_PREDmode, the first neighboring pixels can be as described with respect tothe first neighboring pixels 706 of FIG. 7. For example, when theintra-prediction mode is the SMOOTH_H_PRED mode, the first neighboringpixels can be as described with respect to the first neighboring pixels736 of FIG. 7. For example, when the intra-prediction mode is theSMOOTH_PRED mode, the first neighboring pixels can be as described withrespect to the first neighboring pixels 706 or the first neighboringpixels 736 of FIG. 7.

At 1104, the process 1100 generates second pixels for use along a secondedge that is opposite the first edge of the block. “Generate” as usedherein can mean calculate, determine, select, or otherwise generate inany manner possible. The second pixels are generated using thirdneighboring pixels. The third neighboring pixels are peripheral to athird edge of the block, which is different from the first edge and thesecond edge.

In an example, if the intra-prediction mode is the SMOOTH_V_PRED mode,the second pixels can be as described with respect to the second pixels708 of FIG. 7, and the third neighboring pixels can be as described withrespect to the third neighboring pixels 710 of FIG. 7.

In an example, if the intra-prediction mode is the SMOOTH_H_PRED mode,the second pixels can be as described with respect to the second pixels738 of FIG. 7, and the third neighboring pixels can be as described withrespect to the third neighboring pixels 740 of FIG. 7.

In an example, if the intra-prediction mode is the SMOOTH_PRED mode, thesecond pixels can be as described with respect to the second pixels 708or the second pixels 738 of FIG. 7, and the third neighboring pixels canbe as described with respect to the third neighboring pixels 710 or thethird neighboring pixels 740 of FIG. 7.

At 1106, the process 1100 generates a prediction block that includespredicted pixels. Each of the predicted pixels can be generated byinterpolating the first neighboring pixels and the second pixels toobtain the predicted pixels. The process 1100 uses a first interpolationmethod.

In an example, the process 1100 uses a quadratic interpolation (i.e.,the first interpolation method). The quadratic interpolation can be asdescribed with respect to FIG. 7. In an example, the process 1100 caninclude looking up coefficients for the quadratic interpolation in aweights table. The weights table can be as described with respect to theweights table 810 of FIG. 8.

In an example, and in the case where the intra-prediction mode is theSMOOTH_PRED mode, the process 1100 can include generating fourth pixelsfor use along a fourth edge that is opposite the third edge of theblock. For example, if the second pixels are as described with respectto the pixel 714, then the fourth pixels are as described with respectto the pixel 744, and vice versa. As such, the process 1100 can alsoinclude interpolating, using a second interpolation method, the thirdneighboring pixels and the fourth pixels to obtain the predicted pixels.In an example, the second interpolation method can be the same as thefirst interpolation method (e.g., a quadratic interpolation). In anotherexample, the first interpolation method and the second interpolationmethod can be different. For example, the first interpolation method canbe a linear interpolation, and the second interpolation method can be aquadratic interpolation; or the first interpolation method can be aquadratic interpolation, and the second interpolation method can be alinear interpolation. Other combinations of the first interpolationmethod and the second interpolation method are possible.

In an implementation, the prediction block can include the secondpixels. For example, in the case of the SMOOTH_V_PRED intra-predictionmode, the last row of the prediction block can be set to the secondpixels. That is, instead of determining the pixel values of the last rowof the prediction block by interpolation, the pixels of the last row areset to the second pixels. For example, a row 720 of FIG. 7 can be set tothe pixel 714. As such, if the prediction block includes N rows, thenonly the pixels of the first N−1 rows are generated by interpolation.Accordingly, the prediction block can include the fourth pixels.

Similarly, in the case of the SMOOTH_H_PRED intra-prediction mode, thelast column of the prediction block can be set to the second pixels.That is, instead of determining the pixel values of the last column ofthe prediction block by interpolation, the pixels of the last column areset to the second pixels. For example, a column 750 of FIG. 7 can be setto the pixel 744. As such, if the prediction block includes M columns,then only the pixels of the first M−1 rows are generated byinterpolation.

Similarly, in the case of the SMOOTH_PRED intra-prediction mode, theprediction block can include at least one of the second pixels and thefourth pixels.

In an implementation, the last column of the prediction block can be setto the fourth pixels. That is, instead of determining the pixel valuesof the last row of the prediction block by interpolation, the pixels ofthe last row are set to the second pixels. For example, a row 720 ofFIG. 7 can be set to the pixel 714. As such, if the prediction blockincludes N rows, then only the pixels of the first N−1 rows aregenerated by interpolation. Accordingly, the prediction block caninclude the fourth pixels.

When implemented by an encoder, the process 1100 can include generatinga residual block using the prediction block and the block of video, andencoding the residual block in a compressed bitstream, such as thecompressed bitstream 420 of FIG. 4. The residual block can be generatedas described with respect to the intra/inter-prediction stage 402 toproduce a residual block of FIG. 4. The residual block can be encoded inthe compressed bitstream as described with respect to at least one ofthe transform stage 404, the quantization stage 406, and the entropyencoding stage 408 of the encoder 400. As such, a transform type isselected in the encoding of the residual block.

In some examples, the transform type selected by the encoder can dependon the intra-prediction mode. The transform types available can includean ADST_ADST transform type, a DCT_ADST transform type, and an ADST_DCTtransform type. The ADST_ADST, DCT_ADST, and ADST_DCT transform typesare 2-dimensional transform types. The ADST_ADST transform type appliesan ADST transform to the rows of a transform block followed by anotherADST transform to the columns. The DCT_ADST transform type applies a DCTtransform to the rows of a transform block followed by another ADSTtransform to the columns. The ADST_DCT transform type applies an ADSTtransform to the rows of a transform block followed by a DCT transformto the columns.

In an example, in a case where the intra-prediction mode is theSMOOTH_PRED intra-prediction mode, the transform type can be theADST_ADST transform type. In an example, in a case where theintra-prediction mode is the SMOOTH_H_PRED intra-prediction mode, thetransform type can be the DCT_ADST transform type. In an example, in acase where the intra-prediction mode is the SMOOTH_V_PREDintra-prediction mode, the transform type can be the ADST_DCT transformtype.

When implemented by a decoder, the process 1100 can include decoding aresidual block, and reconstructing the block using the residual blockand the prediction block. The residual block can be decoded as describedwith respect to at least one of the dequantization stage 504 and theinverse transform stage 506 of the decoder 500. As such, a transformtype can be used to decode the residual block. More specifically, theinverse of the transform is used to decode transform blocks.

In an example, the transform type selected can depend on theintra-prediction mode. For example, in a case where the intra-predictionmode is the SMOOTH_PRED intra-prediction mode, the transform type can bethe ADST_ADST transform type, which inverse transforms the rows of atransform block with the ADST transform and the columns of the transformblock with the ADST transform. For example, in a case where theintra-prediction mode is the SMOOTH_H_PRED intra-prediction mode, thetransform type can be the DCT_ADST transform type, which inversetransforms the rows of a transform block with the ADST transform and thecolumns of the transform block with the DCT transform. For example, in acase where the intra-prediction mode is the SMOOTH_V_PREDintra-prediction mode, the transform type can be the ADST_DCT transformtype, which inverse transforms the rows of a transform block with theDCT transform and the columns of the transform block with the ADSTtransform.

For simplicity of explanation, the processes 900, 1000, and 1100 areeach depicted and described as a series of blocks, steps, or operations.However, the blocks, steps, or operations in accordance with thisdisclosure can occur in various orders and/or concurrently.Additionally, other steps or operations not presented and describedherein may be used. Furthermore, not all illustrated steps or operationsmay be required to implement a technique in accordance with thedisclosed subject matter.

The aspects of encoding and decoding described above illustrate someencoding and decoding techniques. However, it is to be understood that“encoding” and “decoding,” as those terms are used in the claims, couldmean compression, decompression, transformation, or any other processingor change of data.

The words “example” or “implementation” are used herein to mean servingas an example, instance, or illustration. Any aspect or design describedherein as “example” or “implementation” is not necessarily to beconstrued as being preferred or advantageous over other aspects ordesigns. Rather, use of the words “example” or “implementation” isintended to present concepts in a concrete fashion. As used in thisapplication, the term “or” is intended to mean an inclusive “or” ratherthan an exclusive “or.” That is, unless specified otherwise or clearlyindicated otherwise by the context, “X includes A or B” is intended tomean any of the natural inclusive permutations thereof. That is, if Xincludes A; X includes B; or X includes both A and B, then “X includes Aor B” is satisfied under any of the foregoing instances. In addition,the articles “a” and “an” as used in this application and the appendedclaims should generally be construed to mean “one or more” unlessspecified otherwise or clear from the context to be directed to asingular form. Moreover, use of the term “an implementation” or “oneimplementation” throughout is not intended to mean the same embodimentor implementation unless described as such.

Implementations of the transmitting station 102 and/or the receivingstation 106 (and the algorithms, methods, instructions, etc., storedthereon and/or executed thereby, including by the encoder 400 and thedecoder 500) can be realized in hardware, software, or any combinationthereof. The hardware can include, for example, computers, intellectualproperty (IP) cores, application-specific integrated circuits (ASICs),programmable logic arrays, optical processors, programmable logiccontrollers, microcode, microcontrollers, servers, microprocessors,digital signal processors, or any other suitable circuit. In the claims,the term “processor” should be understood as encompassing any of theforegoing hardware, either singly or in combination. The terms “signal”and “data” are used interchangeably. Further, portions of thetransmitting station 102 and the receiving station 106 do notnecessarily have to be implemented in the same manner.

Further, in one aspect, for example, the transmitting station 102 or thereceiving station 106 can be implemented using a general-purposecomputer or general-purpose processor with a computer program that, whenexecuted, carries out any of the respective methods, algorithms, and/orinstructions described herein. In addition, or alternatively, forexample, a special-purpose computer/processor, which can contain otherhardware for carrying out any of the methods, algorithms, orinstructions described herein, can be utilized.

The transmitting station 102 and the receiving station 106 can, forexample, be implemented on computers in a video conferencing system.Alternatively, the transmitting station 102 can be implemented on aserver, and the receiving station 106 can be implemented on a deviceseparate from the server, such as a handheld communications device. Inthis instance, the transmitting station 102, using an encoder 400, canencode content into an encoded video signal and transmit the encodedvideo signal to the communications device. In turn, the communicationsdevice can then decode the encoded video signal using a decoder 500.Alternatively, the communications device can decode content storedlocally on the communications device, for example, content that was nottransmitted by the transmitting station 102. Other transmitting station102 and receiving station 106 implementation schemes are available. Forexample, the receiving station 106 can be a generally stationarypersonal computer rather than a portable communications device, and/or adevice including an encoder 400 may also include a decoder 500.

Further, all or a portion of implementations of the present disclosurecan take the form of a computer program product accessible from, forexample, a tangible computer-usable or computer-readable medium. Acomputer-usable or computer-readable medium can be any device that can,for example, tangibly contain, store, communicate, or transport theprogram for use by or in connection with any processor. The medium canbe, for example, an electronic, magnetic, optical, electromagnetic, orsemiconductor device. Other suitable mediums are also available.

The above-described embodiments, implementations, and aspects have beendescribed in order to allow easy understanding of the present disclosureand do not limit the present disclosure. On the contrary, the disclosureis intended to cover various modifications and equivalent arrangementsincluded within the scope of the appended claims, which scope is to beaccorded the broadest interpretation as is permitted under the law so asto encompass all such modifications and equivalent arrangements.

What is claimed is:
 1. A method for coding a block of a video frameusing an intra-prediction mode, comprising: selecting first neighboringpixels, the first neighboring pixels being peripheral to the block alonga first edge of the block; generating second pixels for use along asecond edge that is opposite the first edge of the block, wherein thesecond pixels are generated using third neighboring pixels, the thirdneighboring pixels being peripheral to a third edge of the block, andthe third edge being different from the first edge and the second edge;and generating a prediction block comprising predicted pixels by:interpolating, using a first interpolation, the first neighboring pixelsand the second pixels to obtain the predicted pixels.
 2. The method ofclaim 1, wherein the interpolation is a quadratic interpolation.
 3. Themethod of claim 2, wherein interpolating, using the interpolation, thefirst neighboring pixels and the second pixels comprises: looking upcoefficients for the quadratic interpolation in a weights table.
 4. Themethod of claim 1, further comprising: generating fourth pixels for usealong a fourth edge that is opposite the third edge of the block,wherein the fourth pixels are generated using the first neighboringpixels, wherein generating the prediction block further comprises:interpolating, using a second interpolation, the third neighboringpixels and the fourth pixels to obtain the predicted pixels.
 5. Themethod of claim 4, wherein the prediction block comprises the fourthpixels.
 6. The method of claim 1, wherein the prediction block comprisesthe second pixels.
 7. The method of claim 1, further comprising: codingthe intra-prediction mode, wherein the intra-prediction mode is selectedfrom a first set comprising SMOOTH_PRED, SMOOTH_V_PRED, andSMOOTH_H_PRED; generating a transform block for a residual block;selecting a transform type for coding the residual block from a secondset comprising a DCT_ADST transform type, an ADST_DCT transform type,and an ADST_ADST transform type, wherein in a case where theintra-prediction mode is the SMOOTH_PRED, the transform type is theADST_ADST transform type, in a case where the intra-prediction mode isthe SMOOTH_H_PRED, the transform type is the DCT_ADST transform type,and in a case where the intra-prediction mode is the SMOOTH_V_PRED, thetransform type is the ADST_DCT transform type; and coding the transformblock using the transform type.
 8. An apparatus for encoding a block ofa video frame using an intra-prediction mode, comprising: a memory; anda processor, wherein the processor is configured to execute instructionsstored in the memory to: select first neighboring pixels, the firstneighboring pixels being peripheral to the block along a first edge ofthe block; generate second pixels for use along a second edge that isopposite the first edge of the block, wherein the second pixels aregenerated using third neighboring pixels, the third neighboring pixelsbeing peripheral to a third edge of the block, and the third edge beingdifferent from the first edge and the second edge; generate a predictionblock comprising predicted pixels by executing instructions to:interpolate, using an interpolation, the first neighboring pixels andthe second pixels to obtain the predicted pixels; generate a residualblock using the prediction block and the block of video; and encode theresidual block in a compressed bitstream.
 9. The apparatus of claim 8,wherein the interpolation is a quadratic interpolation.
 10. Theapparatus of claim 9, wherein to interpolate, using the interpolation,the first neighboring pixels and the second pixels comprises: looking upcoefficients for the quadratic interpolation in a weights table.
 11. Theapparatus of claim 8, wherein the instructions further compriseinstructions to: generate fourth pixels for use along a fourth edge thatis opposite the third edge of the block, wherein the fourth pixels aregenerated using the first neighboring pixels, wherein to generate theprediction block further comprises: interpolating, using theinterpolation, the third neighboring pixels and the fourth pixels toobtain the predicted pixels.
 12. The apparatus of claim 11, wherein theprediction block comprises the fourth pixels.
 13. The apparatus of claim8, wherein the prediction block comprises the second pixels.
 14. Theapparatus of claim 8, wherein the instructions further compriseinstructions to: encode the intra-prediction mode, the intra-predictionmode selected from a first set comprising SMOOTH_PRED, SMOOTH_V_PRED,and SMOOTH_H_PRED, wherein to encode the residual block in thecompressed bitstream comprises: selecting a transform type for codingthe residual block from a second set comprising a DCT_ADST transformtype, an ADST_DCT transform type, and an ADST_ADST transform type,wherein in a case where the intra-prediction mode is the SMOOTH_PRED,the transform type is the ADST_ADST transform type, in a case where theintra-prediction mode is the SMOOTH_H_PRED, the transform type is theDCT_ADST transform type, and in a case where the intra-prediction modeis the SMOOTH_V_PRED, the transform type is the ADST_DCT transform type.15. An apparatus for decoding a block of a video frame using anintra-prediction mode, comprising: a memory; and a processor, whereinthe processor is configured to execute instructions stored in the memoryto: select first neighboring pixels, the first neighboring pixels beingperipheral to the block along a first edge of the block; generate secondpixels for use along a second edge that is opposite the first edge ofthe block, wherein the second pixels are generated using thirdneighboring pixels, the third neighboring pixels being peripheral to athird edge of the block, and the third edge being different from thefirst edge and the second edge; generate a prediction block comprisingpredicted pixels by executing instructions to: interpolate, using aninterpolation, the first neighboring pixels and the second pixels toobtain the predicted pixels; decode, from a compressed bitstream, aresidual block; and reconstruct the block using the residual block andthe prediction block.
 16. The apparatus of claim 15, wherein theinterpolation is a quadratic interpolation.
 17. The apparatus of claim16, wherein to interpolate, using the interpolation, the firstneighboring pixels and the second pixels comprises: looking upcoefficients for the quadratic interpolation in a weights table.
 18. Theapparatus of claim 15, wherein the instructions further compriseinstructions to: generate fourth pixels for use along a fourth edge thatis opposite the third edge of the block, wherein the fourth pixels aregenerated using the first neighboring pixels, wherein to generate theprediction block further comprises: interpolating, using theinterpolation, the third neighboring pixels and the fourth pixels toobtain the predicted pixels.
 19. The apparatus of claim 18, wherein theprediction block comprises the fourth pixels.
 20. The apparatus of claim15, wherein the instructions further comprise instructions to: decode,from the compressed bitstream, the intra-prediction mode, theintra-prediction mode being one of SMOOTH_PRED, SMOOTH_V_PRED, andSMOOTH_H_PRED, wherein to decode the residual block from the compressedbitstream comprises: selecting a transform type for decoding theresidual block from a second set comprising a DCT_ADST transform type,an ADST_DCT transform type, and an ADST_ADST transform type, wherein ina case where the intra-prediction mode is the SMOOTH_PRED, the transformtype is the ADST_ADST transform type, in a case where theintra-prediction mode is the SMOOTH_H_PRED, the transform type is theDCT_ADST transform type, and in a case where the intra-prediction modeis the SMOOTH_V_PRED, the transform type is the ADST_DCT transform type.